The present invention relates generally to receivers and, more particularly, to a frame synchronizer for use in a digital receiver.
Communication systems employing digital transmitters and digital receivers are widely used. Such systems, which are commonly employed in mobile communication applications, such as mobile or car telephones, use digital modulation techniques such as binary phase shift keying (BPSK), differential binary phase shift keying (DPSK), quadrature phase shift keying (QPSK) or differential quadrature phase shift keying (DQPSK) and the like. Using these techniques, digital information is transmitted in bursts called frames, which are typically 20 milliseconds (ms) long. Frames have a number of sections or subsections that may range in size from 160 microseconds (xcexcs) to 640 xcexcs and that contain numerous digital symbols, which are transmitted approximately every 40 xcexcs. As is known, digital symbols may be encoded to each represent a number of digital bits. Furthermore, each frame may include a preamble (midamble, etc.) bit sequence that is, for example, 14 bits or 560 xcexcs in length and that is known by each receiver that is to receive the transmitted frame. Each frame also includes data sections having various bits or symbols that represent digitized audio. In some applications, the preamble may be used to address the frame to a particular receiver or receivers while the data sections include the encoded audio (or other) signal for that receiver. As will be appreciated by those familiar with the communication arts, not all of the information in every frame will be received error free because the fidelity of any particular communication system, while it may be high, is not perfect.
Mobile receivers, such as cellular telephones using, for example, time division multiple access (TDMA) standards like the IS-136 standard, are turned on and off on a regular basis and, thus, need to synchronize in both time and frequency to incoming signals to properly decode those signals. In some cases, mobile receivers decode the received signal to produce a bitstream and then perform an auto-correlation procedure on the decoded bitstream to recognize the location of the preamble, which may be designed to have a particularly recognizable auto-correlation function. In other cases, the receiver may perform a cross-correlation procedure between the incoming bitstream and a locally-stored and known preamble to detect the location of the preamble. Generally speaking, the step of decoding the received symbols is prone to error because of both timing and frequency offsets between the receiver and the transmitter. For example, a slight frequency offset between an RF unit of the receiver and an RF unit of the transmitter that transmitted the signal will cause decoding errors. Similarly, sample time offsets between the receiver and the transmitter may lead to the incorrect decoding of symbols, thereby producing erroneous bits in the bitstream.
Bit error rate (BER) is a well-known metric that is used to specify or quantify the fidelity of a digital communication system. BER is a comparison between bits sent over a channel by a digital transmitter and bits received from the channel by a digital receiver. If the received bits are identical to the sent bits, the BER is zero, thus indicating that the communications system including the communication channel, the digital transmitter and the digital receiver has perfect fidelity. Conversely, if the bits received are substantially different from the bits that were sent, the communication system has low fidelity. For example, if there is one bit error in 100 bits, the BER of a particular communication system is 0.01.
Digital communication systems are susceptible to various noise sources that decrease the fidelity of a communication system and, therefore, increase the BER of the communication system. Thermal noise (also called KT noise) is noise resulting from the temperature of various critical components in the digital communication system. Co-channel noise is noise caused by interference on the communication channel over which a digital transmitter is broadcasting. Of particular interest in mobile communication systems is multipath noise.
Multipath noise is noise caused by reception of delayed versions of a previously-received signal because energy from a digital transmitter may take more than one path to a digital receiver. For example, energy from a digital transmitter that takes the most direct path to the receiver arrives at the receiver first, while energy taking another path, such as one with one or more reflections from obstructions, the earth or the atmosphere, arrives at the digital receiver some relatively short time later. Energy that does not take the most direct path from the digital transmitter to the digital receiver is called multipath energy, or simply xe2x80x9cmultipath.xe2x80x9d In a mobile communications system, such as a cellular system, where either a digital receiver or a digital transmitter is moving, the communication path between a transmitter and receiver is constantly changing and, therefore, so is the multipath. For example, as a person using a cellular phone travels in his or her car, multipath may range from nonexistent at one geographic location, to extremely high at another geographic location. Because the multipath is always changing, it is difficult for a digital receiver in a mobile system to synchronize to a digital transmitter in both time and frequency. Typically in a digital system, time synchronization can only be achieved after frequency synchronization is achieved. Accordingly, prior approaches to time and frequency synchronization are relatively time consuming because the synchronization steps must be performed sequentially. A digital receiver such as may be found in a cellular telephone and the like, therefore, may have a significant time delay, due to synchronization operations between when a user attempts to use the receiver and when the receiver is synchronized and ready for use.
A synchronizer for use in a receiver that receives an encoded signal from a transmitter includes a conjugation unit, a delay unit and multiplier coupled together to process the encoded signal. The conjugation unit and the delay unit create both a conjugated version of the received encoded signal and a delayed version of the received encoded signal or create a single conjugated, delayed (e.g., conjugated and then delayed or delayed and then conjugated) version of the received encoded signal. The multiplier then multiplies either the received encoded signal and the conjugated delayed version of the received encoded signal or multiplies the conjugated version of the received encoded signal by the delayed version of the received encoded signal to produce a first product signal. A further multiplier multiplies the first product signal with a locally-stored signal to generate a second product signal. An accumulator accumulates the second product signal over a plurality of bit times to generate an accumulated signal that includes a time offset metric representative of a time synchronization offset between the receiver and the transmitter and that includes a frequency offset metric representative of a frequency synchronization offset between the receiver and the transmitter. A synchronization correction unit generates one or both of a time offset correction and a frequency offset correction from the time offset metric and the frequency offset metric. The receiver may use these corrections to correct offsets in the receiver.
The synchronizer may also or instead include a symbol extractor communicatively coupled to the first multiplier adapted to process the first product signal to generate a first digital bitstream and an exclusive OR (XOR) unit communicatively coupled to the symbol extractor that exclusive ORs the first digital bitstream with a locally-stored digital bitstream to produce an output signal. A second accumulator coupled to the XOR unit counts a number of differences between the first digital bitstream and the locally-stored digital bitstream based on the output signal to produce a second accumulated signal including a further time offset metric representative of the time synchronization offset between the receiver and the transmitter.
According to another aspect of the invention, a method of synchronizing a receiver to a transmitter that transmits an encoded signal spanning multiple bit times includes the steps of multiplying the encoded signal associated with a first bit time by a conjugate of the encoded signal associated with a second bit time to create a first product signal and multiplying the first product signal by a locally-stored signal to create a second product signal. The method also accumulates the second product signal over a plurality of bit times to generate an accumulated signal having a magnitude that corresponds to a time synchronization offset between the receiver and the transmitter and a phase that corresponds to a frequency synchronization offset between the receiver and the transmitter. If desired, the method may adjust the receiver based on at least one of the magnitude and the phase of the accumulated signal.